1. Field of the Invention
The present invention relates to a mass flow meter and a mass flow controller which uses the mass flow meter. More particularly, the present invention relates to a mass flow meter with a self-diagnostics function, and a mass flow controller which uses the mass flow meter.
2. Background
A mass flow meter (mass-flow meter) is widely used, for example, for the purpose of measuring mass flow rate of a process gas supplied into a chamber in a manufacturing process of a semi-conductor. In addition, a mass flow meter is not only used independently as mentioned above, but also used as a part which constitutes a mass flow controller (mass-flow controller) together with other members, such as a flow control valve and a control circuit. In the art, there are various types of mass flow meters, such as a thermal type mass flow meter, a differential pressure type mass flow meter, a hot-wire mass flow meter and a Coriolis mass flow meter, for example. Especially, a thermal type mass flow meter is widely used, since it can precisely measure a mass flow rate of a fluid (for instance, a gas or a liquid) with a comparatively simple configuration.
Generally, a thermal type mass flow meter is constituted by a flow passage through which a fluid flows, a bypass disposed in the middle of the flow passage, a sensor tube which branches from the flow passage at the upstream side of the bypass and joins the flow passage at the downstream side of the bypass, a pair of sensor wires wound around the sensor tube and a sensor circuit which comprises a bridge circuit containing the sensor wires and other resistive elements (as shown, for example, in Japanese Patent Application Laid-Open (kokai) No. 2009-192220). The bypass has a flow resistance to a fluid, and is configured so that a fixed proportion of the fluid which flows through the flow passage branches into the sensor tube.
In the above-mentioned configuration, when the pair of sensor wires generates heat by being impressed a predetermined voltage (or flowed a predetermined current), heat generated from the sensor wires is taken by the fluid which flows through the sensor tube. As a result, the fluid which flows through the sensor tube is heated. At this time, as for the sensor wire on the upstream side, heat is taken by the fluid which has not yet been heated. On the other hand, as for the sensor wire on the downstream side, heat is taken by the fluid which has been already heated by the sensor wire on the upstream side. For this reason, the amount of heat taken from the sensor wire on the upstream side is larger than the amount of heat taken from the sensor wire on the downstream side. As a result, temperature of the sensor wire on the upstream side becomes lower than that of the sensor wire on the downstream side. For this reason, an electrical resistance value of the sensor wire on the upstream side becomes lower than an electrical resistance value of the sensor wire on the downstream side. The larger the mass flow rate of the fluid which flows through the sensor tube becomes, the larger the difference between the electrical resistance values, which results from the temperature difference between the sensor wire on the upstream side and the sensor wire on the downstream side thus generated, becomes.
Changes of the difference of the electrical resistance values between the sensor wire on the upstream side and the sensor wire on the downstream side due to a mass flow rate of the fluid can be detected using a bridge circuit etc., for example. Furthermore, a mass flow rate of the fluid which flows through the sensor tube can be calculated based on the change of the difference of the electrical resistance values between the sensor wires thus detected, and the mass flow rate of the fluid which flows through the flow passage can be calculated based on the mass flow rate of the fluid which flows through the sensor tube (as discussed further herein in detail).
In addition, in this specification, a portion which has a function for measuring a mass flow rate of a fluid flowing through a flow passage is referred to as a “flow sensor unit.” For example, a “flow sensor unit” in a thermal type mass flow meter corresponds to a portion which includes a sensor tube, sensor wires and a power supply for supplying an input signal for making the sensor wires generate heat.
Generally, a mass flow meter including a thermal type mass flow meter as mentioned above is calibrated by measuring a flow rate of a reference fluid (e.g., a reference gas, such as nitrogen gas (N2), for instance) on the basis of another calibrated mass flow meter, immediately after the manufacture thereof, for example. However, while using a mass flow meter, it may become difficult to precisely measure a mass flow rate. For example, a measured value of a mass flow rate may show an abnormal value due to an occurrence of an unexpected abnormal situation, such as a situation where a foreign matter adheres to an inner wall of a sensor tube. However, while using a mass flow meter, it is difficult to detect that such an abnormal situation has occurred.
Then, in the art, a mass flow meter, in which two flow sensor units having completely identical specifications are disposed in series, is known, for example. In such a mass flow meter, each of the flow sensor units has been calibrated by measuring a flow rate of a reference fluid on the basis of another calibrated mass flow meter. Thereafter, when the absolute value of a difference between the intensities of the signals corresponding to a mass flow rate of a fluid outputted from these two flow sensor units (hereafter, may be referred to as a “flow rate deviation”) exceeds a predetermined threshold value t while using the mass flow meter, it is judged that an abnormal situation has occurred in any one of these two flow sensor units.
For example, when a measurement error on measuring a mass flow rate of the same sort of fluid at the same temperature and pressure as those of the fluid used for the calibration is 1.0% or less, the above-mentioned predetermined threshold value t is set as a slightly larger value than 1.0%. Thereby, the existence or non-existence of an occurrence of an abnormal situation can be detected with a high sensitivity. Therefore, when the intensity of a signal corresponding to the mass flow rate of the fluid outputted from either of the abovementioned two flow sensor units shows an abnormal value, this malfunction can be detected immediately. But these prior mass flow meters have proven to be unsatisfactory in use when the fluid type in operation is different from the calibration fluid and/or when operating conditions are different than those that existed during calibration.